Examples of gases containing oxides of nitrogen (hereinafter referred to as NO.sub.x) are exhaust gases from combustion apparatuses such as boilers, nitric acid manufacturing plants, various metal treating processes and other nitrogen oxide generating plants.
In recent years, concern has grown over the so-called photochemical smog. One of the main causes of such photochemical smog is a large quantity of NO.sub.x present in the atmosphere. It is therefore desirable to reduce the quantity of NO.sub.x contained in such exhaust gases and/or to remove NO.sub.x from such exhaust gases.
In combustion apparatuses such as boilers, for example, the NO.sub.x content in the exhaust gas has been reduced conventionally by employment of burners and furnaces of improved design. These methods, however, are not very effective because they allow the reduction of NO.sub.x only within narrow limits both for theoretical and economical reasons.
It is also well known in the art to employ the so-called wet type processes for the removal of NO.sub.x from an exhaust gas, using an alkaline aqueous solution containing sodium hydroxide or sodium sulfite; an aqueous solution of potassium permanganate; an aqueous solution of hypochlorite or chlorite; or an aqueous solution of ferrous salt and sulfurous acid alkali salt (alkali sulfite). The present inventors disclosed in their copending application Ser. No. 517,370, filed Oct. 23, 1974, as a new wet type process, a method for removing nitrogen oxides from a gas containing nitrogen oxides, which is characterized by bringing the nitrogen oxides-containing gas into contact with an aqueous solution containing an organic acid alkali salt and a salt of metal selected from the group consisting of Fe, Co, Ni, Cu and Mn in the presence of a sulfurous acid alkali salt. In the abovementioned wet type processes, no method for effectively treating an absorption solution containing absorbed NO.sub.x has been devised. Accordingly, there is a strong demand for a method which is more efficient in removing NO.sub.x from a gas mixture by effectively treating the absorption solution.